Amorphous alloys (or vitreous alloys or vitrified alloys or non-crystalline alloys or metallic glass or glassy alloys) are generally processed by melt quenching metallic materials employing sufficiently fast cooling rates to avoid the crystallization of the materials' primary and inter-metallic phases. As such, the dimensions of articles formed from amorphous alloys are limited, and the processing conditions may not be favorable for a variety of applications.
There exist a number of U.S. Patents (U.S. Pat. Nos. 5,368,659; and 5,618,359 and 5,032,196) which deal with the development of alloy compositions in which the minimum cooling rate required to obtain a bulk glassy alloy sample is relatively low (typically 1-1000 K/s). Such alloys form bulk glass when cooled at rates above this minimum cooling rate. These alloys crystallize when cooled at rates less than this minimum rate. There is a direct relationship between this minimum cooling rate and the maximum thickness of a component which can be cast in the glassy state. The basic premise of this prior art is that the cooling rate of the alloy liquid must exceed a minimum rate to obtain bulk amorphous metal. It should also be noted that amorphous alloys formed by quenching from the liquid state are also generally called “metallic glass” in order to differentiate them form from amorphous alloys formed by other methods.
There are, in fact, other methods also utilized to form metallic amorphous phases. These processes use extended annealing times for atomic diffusion (W. L. Johnson, Progress in Materials Science, 1986 and U.S. Pat. No. 4,564,396) in the solid state (solid state amorphization), and/or extensive plastic deformation by mechanical milling of powders. These methods also involve the use of thin films or powders, in relatively small quantities. The powders, for example, have to be subsequently consolidated to obtain bulk material. As such, the commercial practice of these “solid state” methods is expensive and impractical.
One noteworthy method of “solid state amorphization” is described in U.S. Pat. No. 4,797,166, which outlines a method to form a partially amorphous phase in metallic alloys by “spontaneous vitrification,” achieved by extended annealing of a crystalline alloy at temperatures below the glass transition temperature of the amorphous alloy. The initial crystalline alloy is stable at high temperatures, and is initially prepared by an annealing treatment at this elevated temperature. The first annealing treatment is followed by a “low temperature annealing” (below the glass transition of the product amorphous alloy). This method suffers from the requirement of very long thermal aging times below the glass transition to produce the amorphous phase from the parent crystalline phase. In addition, the fraction of amorphous phase in the final product is generally not uniform (with the amorphous phase forming preferentially in near surface areas of the sample). As such, this method has never been used commercially.
Accordingly, a need exists for an improved method of forming in-situ composites of metallic alloys comprising an amorphous phase without the use of high-rate quenching.